Religious Texts

Religious texts can't be the ultimate truth. All religions are work of fiction. But why most of our Muslim Brethren believe Islam is the only true religion and everything written in Quran is gospel truth? One of the defence can be because really it is, you read it to know it. Well whatever preaching I have listened in the name of Islam is enough to tell me that its not without its fare share of anomalies like all other religion. Accept it that all religion is same means all religions are equally stupid. Or you tell Its only religion I tell mine is the one from sanatan period and all other religions are descendent of it. (Because I don't find so much radicalised Hindu as much as I find it in Muslim, I abhor all form of religious extremism be it Hindu or Muslim)